Since astronauts first traveled in space, they have referred to Earth as the "Blue Planet." Water, which makes our planet appear blue from space, covers approximately 74% of Earth's surface. Interconnecting oceans compose about 71% of Earth's surface, or 97% of the world's water, while fresh water composes only 3% of the planet's water supply. Without water, life as we know it would not exist on Earth -- even the human body is 65% water! As an archipelago in the Pacific Ocean, 600 miles from the nearest mainland, the quality of the Galapagos waters is integral to the survival of the islands' wildlife.

To understand the relationship between biotic factors and the abiotic factors of pH, salinity and dissolved oxygen within aquatic ecosystems.

To define hydrological cycle, salinity, pH and dissolved oxygen.

To collect and analyze aquatic samples from home and compare them to samples from the Galapagos Islands.

CONCEPTS AND TERMS

All the Earth's water, including ice and glaciers, make up the hydrosphere -- much like the atmosphere is composed of air. The hydrosphere is a huge heat reservoir that stores, absorbs and circulates the radiant energy striking the Earth.

The hydrologic cycle, or water cycle, constantly recycles and purifies the water on our Earth. The major processes involved in the hydrologic cycle are evaporation, transpiration (the evaporation of water from the leaves of plants), precipitation, condensation, percolation and runoff.

Earth's ecosystems are affected by both biotic (living) and abiotic (non-living) factors, and are regulated by the law of tolerance. The law of tolerance states that the existence, abundance and distribution of species depends on the tolerance level of each species to physical and chemical factors.
An ecological principle closely related to the law of tolerance is the limiting factor principle. A limiting factor is any abiotic factor that limits or prevents the growth of a population. Limiting factors in terrestrial ecosystems may include the level of soil nutrients, amount of water, light and temperature. In aquatic ecosystems, major limiting factors include pH, the amount of dissolved oxygen in the water and salinity.

Salinity is defined as the amount of various salt minerals in a given volume of water. The salinity of Earth's oceans is approximately 3.5%, but this number varies depending on many other environmental factors, including air and water temperature. Salinity is usually expressed in parts per million (ppm). Thus, sea water has a salinity of approximately 35,000 ppm. Marine organisms, as well as freshwater organisms, have a specific range of tolerance in terms of salinity. Changes in temperature greatly alter environmental conditions in marine ecosystems, and are especially important with respect to biological impact in coastal areas, which provide habitat to about 90% of all marine species on Earth.

The amount of dissolved oxygen in a body of water is an indication of purity. Organic polluting materials, such as effluent from sewage or waste treatment plants, fertilizer run-off and animal waste, consume oxygen. When high levels of these pollutants exist, dissolved oxygen is decreased, as well as the presence of marine organisms that depend on the oxygen to live. If there are no oxygen-consuming pollutants in water, the concentration of dissolved oxygen will largely be determined by water temperature and salinity. The level of oxygen required by different species varies, but most fish require at least 4.0 milligrams per liter to survive for long periods. Like salinity, dissolved oxygen is expressed in ppm. Generally speaking, warm freshwater fish populations need dissolved oxygen concentrations of not less that 4.0 milligrams/liter (mg/l), while cold water fish species require not less than 5.0 mg/l dissolved oxygen. The scale below shows the range of tolerance for most freshwater fish.

pH describes the relative concentration of positive hydrogen ions (H+) and negative hydroxyl ions (OH-) in a solution. A pH value of 7.0 means that equal concentrations of the above ions are present and the solution is said to be neutral. A pH value below 7.0 means that more hydrogen ions (H+) are present than hydroxyl (OH-) ions and the solution is more acidic; a pH of above 7.0 means the solution contains more hydroxyl ions (OH-) than hydrogen ions (H+) and is more alkaline. Most natural waters have pH values of between 5.0 and 8.5. As plants take in CO2 for photosynthesis in aquatic ecosystems, pH values (and alkalinity) rise. Aquatic animals produce the opposite effect -- as animals take in O2 and give off CO2, the pH (and acidity) is lowered.

NOTE FROM SHERRI: There are a number of tests kits useful for this laboratory activity, ranging in price from $19.95 (for the Rapitest Pond Test Kit, which will test for all three values) to $395 (for the LaMotte Water Pollution Introductory Kit for a full range of aquatic testing, 800-344-3100). All are available from your school's science supply vendor and most have two-day delivery service. Sherri's pick for dissolved oxygen is the CHEMetrics Test Kit, for ease and immediate field results. Artemia, Daphnia, Elodea and Spirogyra can all be obtained inexpensively from any biological supply company. Elodea can also be found at most aquarium supply stores, and Spirogyra can be found in local ponds.

SAFETY PRECAUTIONS

Wear safety goggles

Tie long hair back

Use extreme care when working with open flames, hot liquids, hot plates, chemicals, living materials, glassware and electrical cords/outlets

Follow all laboratory and field safety regulations!

PROCEDURES

Testing for Salinity

Prepare 400 milliliters of NaCl and water in a saturated solution. Your teacher will instruct you in this phase of the experiment. Remember: to saturate, you must add NaCl until no more will dissolve and settling occurs.

Divide the solution into four 100 ml beakers.

Measure the temperature of your room.

Place three beakers on hot plates (a hot plate is best for maintenance of temperature, but Bunsen burners will work as long as you carefully regulate the flame). Set the temperatures at 10°C (50°F), 20°C (68°F) and 30°C (86°F), respectively. (The fourth beaker should remain at room temperature.)

Following the instructions on your salinity test kit (Sherri will be using the salinity test from LaMotte), test the level of salinity in each beaker. Note: Salinity tests must be done quickly to maintain temperature balances.

Record all four salinity values in sequential order (from coolest to warmest) in the Water Salinity Data Chart. Click the "Add My Data" button to add your data and see your graphed results.

Testing for Dissolved Oxygen

Obtain water samples from three different sources (such as a water fountain, bathroom sink, hose, pool, pond, lake, etc.).

Using your dissolved oxygen test kit (Sherri will be taking her samples using CHEMets K-7512 by CHEMetric, Inc.), measure and record dissolved oxygen for each sample.

Use the color comparator (or comparable tool in your test kit) to determine the parts per million of dissolved oxygen in your water samples.

Record the dissolved oxygen value for your three samples in the Dissolved Oxygen Water Analysis Data Chart. Click the "Add My Data" button to sequentially add your data, and to see your graphed results. Data from the Galapagos will be added Monday through Wednesday.

Measuring Alkalinity and Acidity Levels

Obtain water samples from three different sources (such as a water fountain, bathroom sink, hose, pool, pond, lake, etc.).

Using a pH meter, pH paper or a pH test kit, test each sample for pH level (Sherri will be using a pH meter).

Following the directions on your test kit, determine the pH values for each of the three samples.

Record the pH values of your three samples in the pH Water Analysis Data Chart. Click the "Add My Data" button to sequentially add your data, and to see your graphed results. Data from the Galapagos will be added Monday through Wednesday.

Effects of Salinity on Aquatic Organisms

Make a wet mount using the Elodea and Spirogyra. With your microscope on low power, locate each and identify at least one cell that can be seen clearly.

Diagram the cell and note the position of cell contents.

Add a drop of 5% NaCl to the slides. Note the condition of the contents of the cells and diagram each. What changes do you observe? What is your hypothesis on the cell condition after NaCl was added?

Now make two slides of Artemia and Daphnia, using distilled water for the wet mount. Observe and diagram each.

Add the 1% NaCl solution. Observe and record any changes in the specimens or their locomotion.

Repeat with 3% NaCl and then 5% NaCl. Observe any changes in behavior, response and locomotion. Record your observations.

Respond to the following questions:

Did both species behave the same in different solutions?

Which seemed to have the greatest range of tolerance to salinity increases?

Which had the smallest range of tolerance?

How would you characterize the difference in response to salinity increases that each animal had?

CRITICAL THINKING QUESTIONS

Based on your findings, explain the relationship between temperature and salinity.

What is the relationship between living organisms and abiotic factors (pH, salinity and dissolved oxygen)?

How did your values for abiotic factors compare to those of the Galapagos Islands?

How can you account for any differences you noticed between your sample and Sherri's?

List two ways that humans affect abiotic factors within ecosystems.

Scientific American Frontiers
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